The US chief scientist has told the BBC that climate change is now a fact.

Professor John Marburger, who advises President Bush, said it was more than 90% certain that greenhouse gas emissions from mankind are to blame.

The Earth may become "unliveable" without cuts in CO2 output, he said, but he labelled targets for curbing temperature rise as "arbitrary".

His comments come shortly before major meetings on climate change at the UN and the Washington White House.

There may still be some members of the White House team who are not completely convinced about climate change - but it is clear that the science advisor to the President and director of the Office of Science and Technology Policy is not one of them.

In the starkest warning from the White House so far about the dangers ahead, Professor Marburger told the BBC that climate change was unequivocal, with mankind more than 90% likely to blame.

The CO2 accumulates in the atmosphere and there's no end point, it just gets hotter and hotter

Despite disagreement on the details of climate science, he said: "I think there is widespread agreement on certain basics, and one of the most important is that we are producing far more CO2 from fossil fuels than we ought to be.

"And it's going to lead to trouble unless we can begin to reduce the amount of fossil fuels we are burning and using in our economies."

Trouble ahead

This is an explicit endorsement of the latest major review of climate science from the Intergovernmental Panel on Climate Change (IPCC).

Professor Marburger said humanity would be in trouble if we did not stop increasing carbon emissions.

The US sees technologies such as biofuels as the way ahead

"The CO2 accumulates in the atmosphere and there's no end point, it just gets hotter and hotter, and so at some point it becomes unliveable," he said.

Professor Marburger said he wished he could stop US emissions right away, but that was obviously not possible.

US backing for the scientific consensus was confirmed by President Bush's top climate advisor, James Connaughton.

The chair of the White House Council on Environmental Quality told BBC News that advancing technology was the best way to curb the warming trend.

"You only have two choices; you either have advanced technologies and get them into the marketplace, or you shut down your economies and put people out of work," he said.

"I don't know of any politician that favours shutting down economies."

'Arbitrary' targets

Mr Bush has invited leaders of major developed and developing nations to the White House later this month for discussions on a future global direction on climate change.

It will follow a UN General Assembly session on the same issue.

Last week the Asia-Pacific Economic Co-operation forum in Sydney backed the UN climate convention as the right body for developing future global policy.

The European Union wants such a policy to adopt its own target of stabilising temperature rise at or below 2C.

But Mr Marburger said the state of the science made it difficult to justify any particular target.

"It's not clear that we'll be in a position to predict the future accurately enough to make policy confidently for a long time," he said.

"I think 2C is rather arbitrary, and it's not clear to me that the answer shouldn't be 3C or more or less. It's a hunch, a guess."

The truth, he said, was that we just do not know what the 'safe' limit is.